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Drilling and tapping for scope mount

Discussion in 'Gunsmithing and Repairs' started by jaysouth, Jan 2, 2013.

  1. jaysouth

    jaysouth Well-Known Member

    I bought a used Topper 30-30. The barrel is drilled and tapped for 3 screws to fasten the mount on. It appears that two of the holes are boogered out and mount screws are only actually engaging one hole.

    I plan to drill out the holes and re-tap to 8-40.

    What size drill bit do I need for an 8-40 pilot hole. Also, do I need a tapered or plug tap?

    As you can tell, I am not much of a machinist or gunsmity, but this is a pretty simple job that I feel comfortable doing by myself.

  2. BBBBill

    BBBBill Well-Known Member

    Tap drill size for an 8-40 is #28. Are your holes going to be blind or all the way through? If blind, how deep? That will determine which is the appropriate type tap. Also, doing it freehand is likely to give you wallowed out, egg shaped holes which may be what happened the first time. I strongly suggest a drill and tap guide to keep the drill straight. Otherwise it will follow the path of least resistance leaving your new holes as boogered as the old ones.
  3. rcmodel

    rcmodel Member in memoriam

    Listen to what BBBBill said.

    You can't free-hand it and expect it to end well at all!

    If you have a drill press, clamp the gun in a table vice, clamp the vice to the table, drill out the hole, then without moving the gun.
    Chuck the tap in the drill chuck, and turn it in by hand on the chuck to tap the treads.

    That will at least give you a fighting chance of keeping the tap straight with the hole.

    And if the holes are in the barrel as you stated, you for sure have to use a Bottoming tap.
    NOT a Tapered or Plug tap.


    Tap Types and Suggested Uses:
    •Bottom tap has a chamfer (taper at the end) that is 1 to 2 threads long. Use for threading close to the bottom of blind holes.

    •Plug tap has a chamfer that is 3 to 5 threads long. The most common chamfer for use by hand or for machine tapping through blind holes.

    •Taper tap has a chamfer that is 7 to 10 threads long. Easier starting and requires less tapping torque because of the number of working teeth. Also known as a starter tap.

  4. jaysouth

    jaysouth Well-Known Member

    The holes will be drilled out on a drill press with the barrel solidly locked in a machinist's vise.

    If I have problems with keeping the tap perpenduclar to the stock, I will rig a jig to keep it straight.

    Thanks on the advice on the drill bit and tap type
  5. BBBBill

    BBBBill Well-Known Member

    With all of the effort to set it up and drill/tap correctly, please place one thing foremost in your mind. That is to not drill any deeper than you need to go for the task at hand. Holes drilled too close to or all the way through into the bore, chamber, or locking lugs because you got distracted and forgot to set a depth stop is counterproductive unless you are making a paperweight. Don't ask me how I know that. My ego has never healed. :(
    Last edited: Jan 3, 2013
  6. jaysouth

    jaysouth Well-Known Member

    Thanks for the advice. I understand that "compensator holes" in the chamber area are to be avoided.

    Thank goodness I have never done anything like that before:D

    Many thanks for the responses.
  7. Kp321

    Kp321 Well-Known Member

    Brownells sells an oversize tap and screws, slightly larger than #6, for such situations. Still 48 tpi so,less work than re-drilling for #8's.

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